WMG / Geist: The Sin-Eaters

Kerberoi are Wraiths that escaped Oblivion and Gehenna

Kerberoi are obviously ancient creatures tied to death, and possess very broad powers. Perhaps they are just Wraiths that managed to outlive the Old WoD and ended up ruling over the Underworld. As the oldest dead things in existence, they'd certainly get first dibs, and their powers do have some resemblance to those of Wraiths.